1. Field of the Invention
The present disclosure relates to projectors with laser light sources. More specifically, the disclosure relates to illumination modules using multiple laser light sources.
2. Description of the Related Art
Projectors can be designed to use several different types of light sources. Larger, higher power projectors typically use arc lamps as the light source. More compact, lower power projectors can use solid state light sources, most commonly light emitting diodes (LEDs) or lasers. Projectors with laser light sources typically use one laser for each color, and the various laser beams are combined using color selective mirrors, known as dichroic mirrors. Dichroic mirrors reflect one range of wavelengths and transmit another range of wavelengths. The number of dichroic mirrors needed to combine some number (N) of different wavelength lasers is thus equal to N−1.
Referring now to the drawings, FIG. 1 shows the layout 10 of a typical optical system used for combining the light output from, for example, three lasers 10, 11, 12, which emit a laser beam 70 of a desired wavelength in either a continuous or pulsed fashion.
In one embodiment, the three lasers 10, 11, 12 emit a laser beam 70 of three different color wavelengths. The lasers are arranged on three sides of a square prism, often called an “x-cube” 20. The x-cube 20 is formed by gluing together multiple prisms, generally right angled. In a typical arrangement, the right angle prisms have appropriate dichroic coatings 21, 22 on the surfaces such that when assembled together they reflect or transmit the different wavelengths of laser light. A typical arrangement uses a short pass dichroic coating 21 on one side and a long pass dichroic coating 22 on the other side. In this configuration, light from the green wavelength laser 11 is transmitted through both coatings 21 and 22, while the light from the red wavelength laser 10 is reflected from dichroic coating 21 and the light from the blue wavelength laser 12 is reflected from dichroic coating 22. The combined light is then collected by optics such as a collimating lens 4 and a multiple lens array 5.
The x-cube 20 has several disadvantages, including a need for complex dichroic coatings, and assembly of the four right angle prisms with high precision. If the right angle prisms are not precisely sharp, then the area at the center (“center seam”) of the “x” where the four (4) prisms join will not function properly, causing loss of light. The problem of the center seam is more significant as the size of the x-cube decreases. Also, the dichroic coatings 21, 22 have lower performance if all the lasers have the same polarization, a configuration with the transmitted beam in p-polarization and the reflected beams in s-polarization is preferred for the coatings, but is not acceptable for projectors that use polarization based modulators such as liquid crystal on silicon (LCOS).
FIG. 2 shows the layout of another typical optical system 12 for combining the light output from, for example, three lasers 10, 11, 12 with different colors. Here, three lasers are arranged approximately on a common plane with optional collection lenses 41a, 41b and 41c to reduce the laser beam divergence. A mirror 25 and a first and second dichroic mirror 26, 27 are used to combine the beams. The light from laser 12 is reflected off a mirror 25, toward the first dichroic mirror 26. The light from the laser 12 is transmitted by the first dichroic mirror 26, while the light from the second laser 11 is reflected from the first dichroic mirror 26. At that point, the beams from the first and second lasers are combined. The combined beam of the first and second lasers then reflects from a second dichroic mirror 27, while the light from the third laser 10 is transmitted through the second dichroic mirror 27. At that point the light from all three lasers is combined and proceeds toward the rest of the optics, which can include beam shaping and homogenization optical elements such as a collimator lens 4 and a multiple lens array 5.
Individual dichroic mirrors have the disadvantage of creating unequal path lengths, which may create more complicated optical layouts. A dichroic x-cube eliminates the problem of unequal path lengths, but can be difficult to fabricate in small sizes due to the resulting need for a small seam at the center of the x-cube.
Also, dichroic mirrors do not allow the combination of beams from lasers with the same or similar wavelengths.